This invention relates to an acceleration sensor used for measurement of acceleration and for detection of vibration etc. and a method for producing the same. More specifically, this invention relates to a small, quality acceleration sensor and a method for producing the same. Moreover, this invention relates to a shock detecting device using the acceleration sensor, the output of which varies less than other sensors.
Recently electronic devices have been more miniaturized and portable electronics devices including note-type personal computers have been widely used. A small, surface-mountable acceleration sensor with quality are more needed in order to certify the reliability of such electronic devices against shock.
A high-density hard disc can be taken as an example. If the disc is shocked during writing operation, the head is displaced and, as a result, the data cannot be written or the head itself can be damaged. In order to avoid such problems, it is necessary to detect shock to the head, and then stop writing or move the head to a safe position.
Demands for a shock detecting device acceleration sensor for an airbag apparatus are also increased so as to protect a driver from the shock caused by a car collision.
It is also needed to install a configuration in an apparatus which detects shock applied to a portable device and avoids failure or malfunction of the device due to the shock, or records the shock. Therefore, the needs for a small acceleration sensor used for such a device have been also increased.
Acceleration sensors employing piezoelectric materials such as piezoelectric ceramics has been well-known. Such an acceleration sensor can realize a high detection sensitivity by using the electromechanical conversion characteristics of the piezoelectric materials. A piezoelectric acceleration sensor outputs forces caused by acceleration or vibration after converting these forces into voltage by the piezoelectric effect. One example of such an acceleration sensor uses a cantilever structure rectangular bimorph electromechanical transducer as disclosed in Unexamined Japanese Patent Application (Tokkai-Hei) 2-248086. As shown in FIG. 26 of this application, a bimorph electromechanical transducer 50 using the piezoelectric effect is produced by fastening piezoelectric ceramics (51a, 51b) formed with electrodes (52a, 52b) with an adhesive 53 (e.g. epoxy resin). The cantilever structure shown in FIG. 27 is formed by adhering and fixing an end of the electromechanical transducer 50 to a fixing portion 55 with, for example, a conductive adhesive 54. Such a cantilever structure electromechanical transducer having low resonance frequency is used for measurement of acceleration having relatively low frequency components. In order to measure of acceleration in a high frequency region, another type of bimorph electromechanical transducer 50, both of whose ends are fixed to fixing portions 55 with, for example, a conductive adhesive 54, is used (see FIG. 28). The resonance frequency can be relatively raised by fixing both ends of the electromechanical transducer (a structure clamped at both ends).
An acceleration sensor is formed by setting the electromechanical transducer 50 in a package while holding the fixing portion 55 to the inner wall of the package. Electric charge generated at the electrodes (52a, 52b) of the electromechanical transducer 50 is conducted out to outer electrodes via, for example, the conductive adhesive 54.
As mentioned above, adhesives including an epoxy resin are used to adhere the piezoelectric ceramics of conventional acceleration sensors. Young""s modulus of the epoxy resin is 200xc3x9710xe2x88x9212 m2/N, which is bigger than that of the piezoelectric ceramic (150xc3x9710xe2x88x9212 m2/N), so the epoxy resin absorbs the distortion of the electromechanical transducer due to acceleration, and as a result, the sensitivity deteriorates. In addition to that, it is still difficult to adhere piezoelectric ceramics while keeping the thickness of the adhering layer uniform, therefore, the characteristics of the electromechanical transducer will vary.
The resonance frequency of a rectangular bimorph electromechanical transducer should be stable in order to make its sensitivity stable. For this purpose, the electromechanical transducer should be fixed firmly. Actually, however, its metallic supporters or portions supported or fixed by fixing portions will be displaced because of stress generated mechanically or by temperature variation. For instance, if an electromechanical transducer is fixed by using adhesives, the fixing positions will change depending on the adhesive-application range, and thus its resonance frequency will vary. In another case, the fixing condition of the electromechanical transducer will depend on the temperature, so the stable fixing condition is not easily maintained.
In case electromechanical transducers are respectively produced and then set in packages, handling becomes difficult in the producing steps. As a result, the acceleration sensor cannot be miniaturized and quantity production becomes difficult.
The piezoelectric ceramic is produced by mixing and firing several kinds of materials, so, its material constants vary compared to that of a single crystal material. Therefore, sensitivity and capacitance considerably vary.
An acceleration sensor employing piezoelectric ceramics is also used to detect shock on a portable device. Such a device, however, considerably varies in its sensitivity, the standard acceleration range which is set to protect apparatuses from failure tends to be large, and, thus, precise shock detection becomes difficult. Due to the capacitance variation, it is difficult to design a circuit which is connected to the acceleration sensor in order to amplify electric signals generated from acceleration, and, thus, the amplifier degree of the circuit becomes irregular. As a result, the output signal considerably varies, and thus the acceleration sensor cannot reliably be used for shock detecting.
This invention aims to solve the above-mentioned problems of conventional techniques by providing a small acceleration sensor and a method for producing it. This acceleration sensor has high sensitivity in a large frequency region, and its characteristics including sensitivity are remarkably stable. Another purpose of this invention is to provide a shock detecting device using the acceleration sensor, whose output signals are quite stable.
In order to achieve this and other aims, a first acceleration sensor of this invention comprises an electromechanical transducer having a piezoelectric element formed by directly connecting at least two opposite main faces of at least two piezoelectric substrates and electrodes formed on the opposite main faces of the piezoelectric element, and supporters to support the electromechanical transducer. In this first acceleration sensor, the electromechanical transducer is constituted by directly connecting the piezoelectric substrates without using adhesive layers like adhesives. Therefore, if flexible vibration generates in the electromechanical transducer because of acceleration, nothing absorbs the flexible vibration. As a result, the piezoelectric substrates is stressed without loss, a great electromotive force can be obtained, and an acceleration sensor having high sensitivity can be provided. In addition to that, the variations in resonance frequency and sensitivity will considerably be reduced, since the adhesion between the piezoelectric substrates is uniform. Furthermore, the vibration characteristics of the electromechanical transducer do not change due to temperatures, since adhesive layers do not exist between the piezoelectric substrates.
It is preferable in the first acceleration sensor that the main faces of the two piezoelectric substrates are connected by bonding the atoms of the piezoelectric substrates via at least one group selected from the group consisting of oxygen and hydroxyl groups. In this preferred embodiment, the main faces of the two piezoelectric substrates are directly and firmly connected to each other at the atomic level.
It is preferable in the first acceleration sensor that the two piezoelectric substrates are connected to each other so that the directions of polarization axes are opposite to each other. In the preferred embodiment, an electric charge of the same polarity is generated on the two piezoelectric substrates even if the stresses generated in the piezoelectric substrates are different from each other, namely, compressive stress and tensile stress. Electromotive force is generated in the two piezoelectric substrates in the same direction. As a result, signals reflecting the degree of acceleration can be obtained from the electrodes formed on both faces of the electromechanical transducer.
It is preferable in the first acceleration sensor that the two piezoelectric substrates are directly connected via a buffer layer. In this preferred embodiment, strong direct-connecting faces are obtained, since the buffer layer absorbs waviness, irregularities, and foreign materials like contaminants on the adhered faces. In addition, when a material on which oxygen or hydroxyl groups are not easily formed by hydrophilic treatment is used, connection via a buffer layer will provide faces which are strongly and directly connected to each other.
It is preferable in the first acceleration sensor that an end of the electromechanical transducer is supported by supporters. In this preferred embodiment, an acceleration sensor having a cantilever structure can be provided.
It is more preferable in the first acceleration sensor that both ends of the electromechanical transducer are supported by supporters. In this preferred embodiment, an acceleration sensor having a both-ends clamped structure can be provided. This both-ends clamped structure electromechanical transducer enables acceleration measurement in a higher frequency region, since the resonance frequency becomes higher compared to the case of a cantilever structure electromechanical transducer, if the length and thickness are common.
It is preferable in the first acceleration sensor that the piezoelectric substrates comprise single crystal piezoelectric materials of 3 m crystal classes, and that the angle which the main face of the piezoelectric substrates makes with the Y axis is perpendicular to an axis of +129xc2x0 to +152xc2x0 and includes the X axis, and a line which links the center of gravity of the substrates to the center of the supporting portion is perpendicular to the X axis, where the X axis, Y axis and Z axis are the crystal axes of the single crystal piezoelectric materials. In this preferred embodiment, the piezoelectric constant of the piezoelectric substrates is 90 to 100% of the maximum value, and thus problems due to deterioration in sensitivity will not be found.
It is preferable in the first acceleration sensor that the piezoelectric substrates comprise single crystal piezoelectric materials of 3 m crystal classes, and that the angle which the main face of the piezoelectric substrates makes with the Y axis is perpendicular to an axis of xe2x88x9226xc2x0 to +26xc2x0 and includes the X axis, and a line which links the center of gravity of the substrates to the center of the supporting portion is parallel to the X axis, where the X axis, Y axis and Z axis are the crystal axes of the single crystal piezoelectric materials. In this preferred embodiment, the piezoelectric constant of the piezoelectric substrates is 90 to 100% of the maximum value, and thus problems due to deterioration in sensitivity will not be found.
It is preferable in the first acceleration sensor that the piezoelectric substrates comprise single crystal piezoelectric materials of single crystal 32 crystal classes, and that the main face of the piezoelectric substrate is perpendicular to the X axis while a line which links the center of gravity of the substrates to the center of the supporting portion makes an angle of from +52xc2x0 to +86xc2x0 with the Z axis, where the X axis, Y axis and Z axis are the crystal axes of the single crystal piezoelectric materials. In this preferred embodiment, the piezoelectric constant of the piezoelectric substrates is 90 to 100% of the maximum value, and thus problems due to deterioration in sensitivity will not be found.
It is preferable in the first acceleration sensor that the piezoelectric substrates comprise single crystal piezoelectric materials of 32 crystal classes, and that the angle which the main face of the piezoelectric substrates makes with the X axis is perpendicular to an axis of xe2x88x9226xc2x0 to +26xc2x0 and includes the Y axis, and a line which links the center of gravity of the substrates to the center of the supporting portion is parallel to the Y axis, where the X axis, Y axis and Z axis are the crystal axes of the single crystal piezoelectric materials. In this preferred embodiment, the piezoelectric constant of the piezoelectric substrates is 90 to 100% of the maximum value, and thus problems due to deterioration in sensitivity will not be found.
It is preferable in the first acceleration sensor that the piezoelectric substrates comprise single crystal piezoelectric materials of 32 crystal classes, and that the angle which the main face of the piezoelectric substrates makes with the X axis is perpendicular to an axis of +52xc2x0 to +68xc2x0 and includes the Z axis, and a line which links the center of gravity of the substrates to the center of the supporting portion is perpendicular to the Z axis, where the X axis, Y axis and Z axis are the crystal axes of the single crystal piezoelectric materials. In this preferred embodiment, the piezoelectric constant of the piezoelectric substrates is 90 to 100% of the maximum value, and thus problems due to deterioration in sensitivity will not be found.
A second acceleration sensor of this invention comprises an electromechanical transducer having a piezoelectric element formed by connecting two opposite main faces of at least two piezoelectric substrates and electrodes formed on the two opposite main faces, and supporters to support the electromechanical transducer, where the electromechanical transducer is directly connected to the supporters. In this second acceleration sensor, the electromechanical transducer is produced by directly connecting the piezoelectric substrates without using adhesive layers. Therefore, the supporting position of the electromechanical transducer is less varied, and thus, an acceleration sensor having less varied a resonance frequency can be provided. In addition to that, acceleration can be transferred to the electromechanical transducer without loss, since the electromechanical transducer is directly connected to the supporters without using adhesives. Furthermore, the supporting condition will not change with temperature, since adhesive layers do not exist between the piezoelectric substrate and the supporters.
It is preferable in the second acceleration sensor that the piezoelectric substrates and the supporters are connected to each other by bonding the atoms composing the substrates and supporters via at least one group selected from the group consisting of oxygen and hydroxyl groups.
It is preferable in the second acceleration sensor that the piezoelectric substrates composing the electromechanical transducer and the supporters are directly bonded via a buffer layer.
It is also preferable in the second acceleration sensor that the piezoelectric substrates and the supporters compose the same materials. In this preferred embodiment, an acceleration sensor which is extremely stable under temperature variation can be provided, since it is not effected by distortion due to temperature.
It is preferable in the second acceleration sensor that an end of the electromechanical transducer is supported by the supporters.
It is more preferable in the second acceleration sensor that both ends of the electromechanical transducer are supported by the supporters.
A third acceleration sensor of this invention comprises an electromechanical transducer having a piezoelectric element formed by directly connecting two opposite main faces of at least two piezoelectric substrates and electrodes formed on the two opposite main faces, supporters to support the electromechanical transducer, and packages containing the electromechanical transducer, where the supporters are directly connected to the packages. In this third acceleration sensor, the supporters of the electromechanical transducer are directly connected to the packages without using adhesives, so the supporters are strongly connected to the packages. Therefore, a highly sensitive acceleration sensor in which acceleration generated on its mounting surface can be transferred to the supporters without loss via the packages.
It is preferable in the third acceleration sensor that the packages and the supporters are connected by bonding the atoms composing the packages and the supporters via one group selected from the group consisting of oxygen and hydroxyl groups.
It is preferable in the third acceleration sensor that the packages and the supporters are directly connected via a buffer layer.
It is also preferable in the third acceleration sensor that the packages and the supporters be comprised of the same materials.
A fourth acceleration sensor of this invention comprises an electromechanical transducer having a piezoelectric element formed by directly connecting two opposite main faces of at least two piezoelectric substrates and electrodes formed on the two opposite main faces, supporters to support the electromechanical transducer, and packages containing the electromechanical transducer, where the electromechanical transducer is supported by directly connecting the piezoelectric substrates composing the piezoelectric element to the packages. In this fourth acceleration sensor, the piezoelectric substrates are directly connected to the packages without using adhesives, so the electromechanical transducer is strongly connected to the packages. As a result, a highly sensitive acceleration sensor can be obtained since the acceleration the packages receive is transferred to the electromechanical transducer without loss. In addition to that, the sections of the acceleration sensor can be reduced since the packages function as the supporters.
It is preferable in the fourth acceleration sensor that the piezoelectric substrates and the packages are connected by bonding the atoms composing the piezoelectric substrates and the packages via one group selected from the group consisting of oxygen and hydroxyl groups.
It is preferable in the fourth acceleration sensor that the piezoelectric substrates and the packages are directly connected via a buffer layer.
It is also preferable in the fourth acceleration sensor that the piezoelectric substrates and the packages be comprised of the same materials.
It is preferable in the fourth acceleration sensor that a conductive layer is provided to the piezoelectric element excepting the electromechanical transducer. In this preferred embodiment, outer electrodes can be formed on the faces opposite to the supporting portions even if the acceleration sensor has a cantilever structure. In this preferred embodiment, it is also possible to provide electrodes on all of the faces of the electromechanical transducer, and thus a highly sensitive acceleration sensor can be provided.
A fifth acceleration sensor of this invention comprises an electromechanical transducer having a piezoelectric element formed by directly connecting two opposite main faces of at least two piezoelectric substrates and electrodes formed on the two opposite main faces, supporters to support the electromechanical transducer, and packages being composed of at least two parts to contain the electromechanical transducer, where the two parts composing the packages are directly connected to each other. In this fifth acceleration sensor, the parts composing the packages are strongly connected to each other without using adhesives, so the heat resistance characteristics of the connected faces are improved. Therefore, the connecting parts do not generate gases even if solder reflow is conducted, and thus every part composing the packages is air-tight sealed. As a result, a reliable acceleration sensor whose characteristics do not deteriorate can be obtained.
It is preferable in the fifth acceleration sensor that the parts composing the packages are connected to each other by bonding their atoms via one group selected from the group consisting of oxygen and hydroxyl groups.
It is preferable in the fifth acceleration sensor that the parts composing the packages are directly connected to each other via a buffer layer.
In the first method of this invention for producing an acceleration sensor comprising an electromechanical transducer having piezoelectric element formed by connecting two opposite main faces of at least two piezoelectric substrates and electrodes formed on the opposite main faces of the piezoelectric element, and supporters to support the electromechanical transducer, wherein the piezoelectric element is formed by directly connecting the main faces of the two piezoelectric substrates, the electromechanical transducer is formed by directly connecting the piezoelectric substrates without using adhesive layers. As a result, an acceleration sensor where flexible vibration generated due to acceleration at its electromechanical transducer is not absorbed can be provided.
It is preferable in the first method that the main faces of the two piezoelectric substrates are directly connected by heat-treating after the piezoelectric substrates are hydrophilically treated and their main faces are connected to each other. In this preferred embodiment, the main faces of the two piezoelectric substrates are bonded firmly and directly at the atomic level via oxygen or hydroxyl groups.
In the second method of this invention for producing an acceleration sensor comprising an electromechanical transducer having piezoelectric element formed by connecting two opposite main faces of at least two piezoelectric substrates and electrodes formed on the opposite main faces of the piezoelectric element, and supporters to support the electromechanical transducer, wherein the piezoelectric element is formed by directly connecting the supporters to the piezoelectric substrates composing the piezoelectric element, the electromechanical transducer is directly connected to the supporters without using adhesives, and, thus, the variation of the supporting position of the electromechanical transducer is reduced. As a result, an acceleration sensor whose resonance frequency varies less can be obtained.
It is preferable in the second method that the supporters and the piezoelectric substrates are directly connected by heat-treating after the supporters and the piezoelectric substrates are hydrophilically treated and connected to each other.
In the third method of this invention for producing an acceleration sensor comprising an electromechanical transducer having piezoelectric element formed by connecting opposing main faces of at least two piezoelectric substrates and electrodes formed on the opposing main faces of the piezoelectric element, supporters to support the electromechanical transducer, and packages to contain the electromechanical transducer, wherein the supporters are connected to the package directly and firmly, acceleration generated on the mounting surface can be transferred to the supporters via the packages without loss, and a high sensitive acceleration sensor can be obtained.
It is preferable in the third method that the supporters are directly connected to the packages by heat-treating after the supporters and the packages are hydrophilically treated and connected to each other.
In the fourth method of this invention for producing an acceleration sensor comprising an electromechanical transducer having piezoelectric element formed by connecting two opposite main faces of at least two piezoelectric substrates and electrodes formed on the opposite main faces of the piezoelectric element, and packages to contain the electromechanical transducer, wherein the piezoelectric substrates composing the piezoelectric element are directly connected to the packages, the electromechanical transducer can be connected firmly to the packages. As a result, acceleration the packages receive can be transferred to the electromechanical transducer without loss, and a highly sensitive acceleration sensor can be obtained. In addition to that, the sections of the acceleration sensor can be reduced since the packages function as the supporters, and the production process can be simplified.
It is preferable in the fourth method that the piezoelectric substrates are directly connected to the packages by heat-treating after the piezoelectric substrates and the packages are hydrophilically treated and connected to each other.
In the fifth method of this invention for producing an acceleration sensor comprising an electromechanical transducer having a piezoelectric element formed by connecting two opposite main faces of at least two piezoelectric substrates and electrodes formed on the opposite main faces of the piezoelectric element, supporters to support the electromechanical transducer, and packages comprising at least two parts in order to contain the electromechanical transducer, wherein the parts of the packages are directly connected to each other, the parts composing the packages are firmly connected to each other without using adhesives, and thus the heat resistance properties of the connected faces are improved. As a result, gases are not generated from the connected part even if solder reflow is conducted, and each parts of the packages are air-tight sealed. Therefore, a reliable acceleration sensor whose characteristics do not deteriorate can be obtained.
It is preferable in the fifth method that the parts of the packages are directly connected to each other by heat-treating after the parts of the packages are hydrophilically treated and connected to each other.
A sixth method of this invention for producing an acceleration sensor comprising an electromechanical transducer having a piezoelectric element formed by connecting two opposite main faces of at least two piezoelectric substrates and electrodes formed on the opposite main faces of the piezoelectric element, and packages to contain the electromechanical transducer comprising the following processes:
forming plural piezoelectric elements by directly connecting at least two piezoelectric substrates on which plural cantilevers or both-ends clamped structures are pattern-formed;
directly connecting the packages to the piezoelectric substrates, the packages being formed with concavities to correspond to the piezoelectric elements; and
separating the acceleration sensor into individual acceleration sensors containing the piezoelectric elements. In this sixth method, the electromechanical transducers are pattern-formed from the piezoelectric substrates, and thus, the shape of the electromechanical transducer is less varied. Since the electromechanical transducer and the supporters are formed simultaneously, the supporting condition of the electromechanical transducer is quite stable. Therefore, the cantilever or a beam clamped on both ends does not vary much in length, and as a result, an acceleration sensor which has very little variation in characteristics, such as resonance frequency, can be obtained. In addition, the materials of the electromechanical transducer, the supporters and the packages are the same, so an extremely stable acceleration sensor can be provided without the influence of distortion due to temperature. Another advantage of this embodiment is superior quantity productivity, since numbers of acceleration sensors can be produced at a time on a substrate.
It is preferable in the sixth method of this invention that electrodes are formed on the two opposite main faces of the piezoelectric element after the formation of the piezoelectric element. In this preferred embodiment, a mask can be located easily when the electrodes are formed, and also the electrodes can be formed on the piezoelectric elements precisely, since the piezoelectric elements are already formed. As a result, a precise electromechanical transducer can be provided. In this case, it is also preferable that conductive layer be formed on the piezoelectric substrate when the electrodes are formed on the two opposite main faces of the piezoelectric elements, so that the producing processes can be simplified.
It is further preferable in the sixth method of this invention that a cantilever or a beam clamped on both ends is pattern-formed after the electrodes are formed on the piezoelectric substrates. In this preferred embodiment, an electromechanical transducer can be produced without positioning the electrodes precisely. If a piezoelectric element is thin, a short-circuit of the front and back electrodes can occur as the electrodes are formed after the cantilever is pattern-formed. In this preferred embodiment, however, such a problem can be prevented. It is preferable in this embodiment that a conductive layer is formed on the piezoelectric substrate when the electrodes are formed.
A seventh method of this invention for producing an acceleration sensor comprising an electromechanical transducer having a piezoelectric element formed by connecting two opposite main faces of at least two piezoelectric substrates and electrodes formed on the opposite main faces of the piezoelectric element, and packages to contain the electromechanical transducer comprises the following processes:
forming plural piezoelectric elements by pattern-forming plural cantilevers or a beam having both-ends structure after directly connecting at least two piezoelectric substrates;
directly connecting the packages to the piezoelectric substrates, the packages being formed with concavities to correspond to the piezoelectric elements; and
separating the acceleration sensor into individual acceleration sensors containing the piezoelectric elements.
It is preferable in the seventh method of this invention that electrodes are formed on the two opposite main faces of the piezoelectric element after the formation of the piezoelectric element. In this case, it is also preferable that a conductive layer is formed on the piezoelectric substrates when the electrodes are formed on the two opposite main faces of the piezoelectric elements, so that the producing processes can be simplified.
It is further preferable in the seventh method of this invention that a cantilever or a beam clamped on both ends is pattern-formed after the electrodes are formed on the piezoelectric substrates. It is preferable in this embodiment that a conductive layer is formed on the piezoelectric substrate when the electrodes are formed.
A shock detecting device of this invention comprises:
an acceleration sensor provided with an electromechanical transducer comprising piezoelectric elements configured by connecting two opposite main faces of at least two piezoelectric substrates and electrodes formed on the main faces, and supporters to support the electromechanical transducer;
an amplifier circuit which converts and amplifies signals from the acceleration sensor;
a comparator circuit which compares the signals from the amplifier circuit with standard signals;
a control circuit which controls the apparatus in which the acceleration sensor is included; and
a storage device to store shock. The shock detecting device can measure acceleration precisely since there is no variation in sensitivity of the acceleration sensor or in its capacitance. Therefore, the shock detecting device can detect and analyze shocks precisely by using the comparator circuit depending on the standard values, while it can also instruct recording of the detected shock and protect the apparatus from the shock by analysis at its control circuit.